Fuel efficiency of many types of internal combustion engines can be substantially improved by varying the displacement of the engine. This allows for the full torque to be available when required, yet can significantly reduce pumping losses and improve thermodynamic efficiency by using a smaller displacement when full torque is not required. The only commercially available method of varying the displacement today is deactivating a group of cylinders substantially simultaneously. In this approach no fuel is delivered to the deactivated cylinders and their associated intake and exhaust valves are kept closed as long as the cylinders remain deactivated. For example, an 8 cylinder variable displacement engine may deactivate half of the cylinders (i.e. 4 cylinders) so that it is operating using only the remaining 4 cylinders. Commercially available variable displacement engines available today typically support only two or at most three fixed mode displacements.
Another engine control approach that varies the effective displacement of an engine is referred to as “skip fire” engine control. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, a particular cylinder may be fired during one engine cycle and then may be skipped during the next engine cycle and then selectively skipped or fired during the next. In this manner, even finer control of the effective engine displacement is possible. For example, firing every third cylinder in a 4 cylinder engine would provide an effective displacement of ⅓rd of the full engine displacement, which is a fractional displacement that is not obtainable by simply deactivating a set of cylinders. Conceptually, virtually any effective displacement can be obtained using skip fire control, although in practice many implementations restrict operation to a set of available firing fractions, sequences or patterns. The applicant has filed a number of patents describing various approaches to skip fire control.
When operating in skip fire mode, it is generally desirable to deactivate cylinders during skipped working cycles in the sense that air is not passed through the cylinder during the skipped working cycle as it would be if the intake and exhaust valves were opened in the normal manner. Deactivating cylinders during skipped working cycles tends to improve fuel efficiency (due to reduced pumping losses) and improve emissions—since many emission control systems (e.g., catalytic converters) are not designed to handle large amount of air. Therefore, when implementing skip fire control it is desirable to control the intake and exhaust valves in a more complex manner than if the cylinders are always activated or deactivated in sets for prolonged periods of time as occurs with conventional variable displacement engines.
If a valve doesn't activate or deactivate when intended, the performance of the engine can be adversely affected. Engine performance can also be adversely affected if the valve does not open in the manner expected (e.g., if the dwell time of a valve opening is different than expected or the valve lift height is different than expected). Therefore, regardless of the valve actuation strategy employed, it is desirable to be able to verify that the valves have operated as desired and to identify valve actuation faults for both control and diagnostics purposes.
The Applicant has developed a number of approaches for identifying whether an exhaust valve and/or an intake valve has opened or remains closed in particular circumstances. The described techniques can be used independently, or in any desired combination to monitor valve operation and/or to identify valve actuation faults. The resulting knowledge can be used to help manage, diagnose and/or control skip fire operation of an engine and/or to take remedial action when valve actuation faults are detected.